1. Field of the Invention
This invention relates generally to a high frequency amplifier, and more particularly to a high frequency amplifier with a low output impedance.
2. Description of the Prior Art
In a prior art high frequency amplifier, a resonance circuit consisting of a coil and a condenser is connected to the output terminal of a transistor for amplifying and an amplified signal is derived from the resonance circuit.
When another circuit with low input impedance is connected to the high frequency amplifier, it is necessary that a secondary coil is provided in connection with the coil of the resonance circuit to convert the output impedance of the high frequency amplifier to a low impedance.
The prior art high frequency amplifier with the construction mentioned above has the following defect. That is, the resonance frequency of the resonance circuit in the amplifier varies in accordance with the change of an input impedance of a circuit which may be connected to the following stage of the high frequency amplifier.
Further, in the case where the amplifier is formed as an integrated circuit, Q of the resonance circuit is lowered, so that a sufficient selectivity can not be obtained.
Further, due to the impedance conversion, a secondary coil is required for the resonance circuit which is unsuitable when it is formed as an integrated circuit.